This invention concerns the recovery of hydrocarbons from an underground formation by the use of horizontal injection and production wells. More particularly, the invention employs horizontal production and injection wells arranged in a substantially triangular pattern with certain injection and production steps.
Horizontal wells have been investigated and tested for oil recovery for quite some time. Although horizontal wells may in the future be proven economically successful to recover petroleum from many types of formations, at present, the use of horizontal wells is usually limited to formations containing high viscous crude. It seems likely that horizontal wells will soon become a chief method of producing tar sand formations and other highly viscous oils which cannot be efficiently produced by conventional methods because of their high viscosity. Most heavy oil and tar sand formations cannot be economically produced by surface mining techniques because of their formation depth.
Various proposals have been set forth for petroleum recovery with horizontal well schemes. Most have involved steam injection or in situ combustion with horizontal wells serving as both injection wells and producing wells. Steam and combustion processes have been employed to heat viscous formations to lower the viscosity of the petroleum as well as to provide the driving force to push the hydrocarbons toward a well.
A system of using parallel horizontal wells drilled laterally from subsurface tunnels into the lower portion of a tar sand formation is disclosed in U.S. Pat. No. 4,463,988. The described process injects a displacing means such as steam into the boreholes to cause hydrocarbons to flow into the lower portion of the lateral borehole and be produced to the surface.
U.S. Pat. No. 4,577,691 discloses a plurality of parallel horizontal wells arranged in a vertical plane whereby a thermal fluid can be injected into upper wells to drive hydrocarbons down from the area of the upper wells to the horizontal wells immediately below and lying in the same vertical plane. U.S. Pat. No. 4,700,779 discloses a pattern of four or more horizontal wells lying parallel to each other in a horizontal plane within a thin reservoir. The wells in a horizontal plane are used with a combination steam and water injection process to sweep oil from one end to the other end of the pattern.
The use of two or more parallel horizontal injection and production wells is disclosed in U.S. Pat. No. 4,598,770. In this reference, two horizontal wells are drilled parallel to each other at the bottom of the hydrocarbon formation. A thermal fluid is injected through one of the horizontal wells and that fluid and hydrocarbons are produced at the other parallel horizontal well.